<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>异步遍历器 | 知识库演示</title>
    <meta name="generator" content="VuePress 1.9.9">
    <link rel="icon" href="/vdoing-demo-repository/img/favicon.ico">
    <meta name="description" content="web前端技术博客,简洁至上,专注web前端学习与总结。JavaScript,js,ES6,TypeScript,vue,python,css3,html5,Node,git,github等技术文章。">
    <meta name="keywords" content="前端博客,个人技术博客,前端,前端开发,前端框架,web前端,前端面试题,技术文档,学习,面试,JavaScript,js,ES6,TypeScript,vue,python,css3,html5,Node,git,github,markdown">
    <meta name="theme-color" content="#11a8cd">
    
    <link rel="preload" href="/vdoing-demo-repository/assets/css/0.styles.bb913129.css" as="style"><link rel="preload" href="/vdoing-demo-repository/assets/js/app.52e6b5d0.js" as="script"><link rel="preload" href="/vdoing-demo-repository/assets/js/2.f10ddeba.js" as="script"><link rel="preload" href="/vdoing-demo-repository/assets/js/111.84bc1774.js" as="script"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/10.82e086d6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/100.2570dab9.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/101.b429c13f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/102.0e9afba4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/103.42e8d9d6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/104.6ca089b8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/105.f02335fc.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/106.2fe52c32.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/107.7650f18b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/108.7d825011.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/109.42092f8e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/11.49ec595f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/110.8e6f8d5b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/112.5da6cb16.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/113.2eb6714d.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/114.dbdf6a79.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/115.96dc61ac.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/116.2774e841.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/117.5a0ea935.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/118.72005b7a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/119.77fd4a33.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/12.66e2368f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/120.994b6190.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/121.794bec47.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/122.239044f6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/123.d6247b89.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/124.357cddf4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/125.b27c8c6b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/126.2f09d5b4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/127.5b5e7dcc.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/128.ca8342ed.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/129.62196c04.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/13.01d28bf6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/130.ae36e0a1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/131.63408c4f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/132.7a1bc472.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/133.6f365b21.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/134.7dfd70a1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/135.c110453c.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/136.dea9b77b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/137.212998a2.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/138.cf96d5b3.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/139.34437df0.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/14.52e25550.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/140.b28a1de4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/141.d117632d.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/142.743bfbfc.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/143.d6c926e2.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/144.86916e4e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/145.a4658544.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/146.9f190393.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/147.b53beefe.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/148.d49c882d.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/149.b5c8b142.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/15.ff9c9117.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/150.ab4902b7.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/151.aa13cf1a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/152.ca32ca39.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/153.f3dc7a9e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/154.2198d119.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/155.6cf21591.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/156.929b9e50.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/157.6d227e19.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/158.ad6e937f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/159.d74c55e1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/16.a59e9d0e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/160.1e9abdb1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/161.5343ce3e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/162.214b3f61.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/163.918d2361.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/164.204cab40.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/165.884ba632.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/166.e3bc3126.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/167.b8103188.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/168.176b21b8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/169.56e67ae6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/17.73f27920.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/170.05fe9454.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/171.ac72af48.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/172.134bd8be.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/173.f7b83d9b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/174.bc7aa46f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/175.c654668c.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/176.33b09327.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/177.97d7e55f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/178.ab02c611.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/179.0d88c6f5.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/18.39e882fe.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/180.1a2df1f3.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/181.533495f7.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/182.a4fc6807.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/183.e3a44f30.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/184.b731d331.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/185.e6eb67ad.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/186.c2493cb4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/187.279befae.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/188.1de7c987.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/189.e748958a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/19.ef7490a3.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/190.ad045bc8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/191.c3131762.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/192.29ea4e7e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/193.1e75aa14.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/194.f1150906.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/195.2a593c50.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/196.5aa5efe2.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/197.a328dc59.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/198.9db60424.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/199.d3b39a54.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/20.b5301937.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/200.a9dae555.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/21.2e9bd506.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/22.02e5e5a0.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/23.a00bf08d.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/24.dae9e7bf.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/25.3120ccd4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/26.628f5a86.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/27.d4e86439.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/28.642a9f76.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/29.6c013ead.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/3.c3e96c9f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/30.4e45e23b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/31.9b0c4fcb.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/32.347f1a82.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/33.a17e9f72.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/34.ae475141.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/35.70d7eb87.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/36.0166a619.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/37.8d63783a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/38.91c94a9e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/39.edf5fef6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/4.f05e47c1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/40.f2dfda35.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/41.488faa5e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/42.258fde58.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/43.c46a30da.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/44.cfac4a51.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/45.58439e5f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/46.91d42cf4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/47.39ec1b7a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/48.900b1d2a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/49.4db4a295.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/5.85821e9b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/50.4a48e245.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/51.32253e74.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/52.f9692c89.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/53.f93fad53.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/54.cccf7ffb.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/55.7e4f324a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/56.5d04b9d7.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/57.ed34f737.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/58.9e762eed.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/59.81cfbcc1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/6.5f8d8bd6.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/60.76f14de8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/61.a4d207e0.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/62.fce4471e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/63.58fd2153.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/64.b3f081c8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/65.dbde37b1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/66.49bc7970.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/67.48b689d5.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/68.a3660c81.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/69.b4057547.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/7.4acc3e20.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/70.5f3717cf.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/71.d47ff342.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/72.eca3c6ef.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/73.71bbaeec.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/74.6fe6e554.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/75.4dbad835.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/76.6ec52a3f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/77.9041350a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/78.e0b3aa97.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/79.1dbdb52c.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/8.503d9748.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/80.568b41f1.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/81.9394cc64.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/82.e36a0a2e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/83.fba15f4a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/84.1a6a64b4.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/85.af0f0d14.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/86.aed3f80d.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/87.08c34bee.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/88.52b2750e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/89.6d27f363.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/9.9691b37e.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/90.cccb0e61.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/91.6fb1e7f3.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/92.9e3d1d8a.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/93.25bb0c7b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/94.0a896da5.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/95.41991c9b.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/96.1f4103d8.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/97.97ec058f.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/98.f40afd31.js"><link rel="prefetch" href="/vdoing-demo-repository/assets/js/99.ac67e72d.js">
    <link rel="stylesheet" href="/vdoing-demo-repository/assets/css/0.styles.bb913129.css">
  </head>
  <body class="theme-mode-light">
    <div id="app" data-server-rendered="true"><div class="theme-container sidebar-open have-rightmenu"><header class="navbar blur"><div title="目录" class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/vdoing-demo-repository/" class="home-link router-link-active"><img src="/vdoing-demo-repository/img/EB-logo.png" alt="知识库演示" class="logo"> <span class="site-name can-hide">知识库演示</span></a> <div class="links"><div class="search-box"><input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/vdoing-demo-repository/" class="nav-link">首页</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="Python" class="dropdown-title"><a href="/vdoing-demo-repository/python/" class="link-title">Python</a> <span class="title" style="display:none;">Python</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>Python</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/c438df/" class="nav-link">Python基础</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/vdoing-demo-repository/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/802a1ca6f7b71c59/" class="nav-link">Vue</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/javascript/" class="nav-link">《JavaScript教程》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/es6/" class="nav-link">《ES6 教程》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/vue/" class="nav-link">《Vue》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/wx-miniprogram/" class="nav-link">小程序笔记</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/vdoing-demo-repository/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/vdoing-demo-repository/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><a href="/vdoing-demo-repository/archives/" class="nav-link">归档</a></div> <a href="https://github.com/xugaoyi/vuepress-theme-vdoing" target="_blank" rel="noopener noreferrer" class="repo-link">
    GitHub
    <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav></div></header> <div class="sidebar-mask"></div> <div class="sidebar-hover-trigger"></div> <aside class="sidebar" style="display:none;"><!----> <nav class="nav-links"><div class="nav-item"><a href="/vdoing-demo-repository/" class="nav-link">首页</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="Python" class="dropdown-title"><a href="/vdoing-demo-repository/python/" class="link-title">Python</a> <span class="title" style="display:none;">Python</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>Python</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/c438df/" class="nav-link">Python基础</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/vdoing-demo-repository/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/pages/802a1ca6f7b71c59/" class="nav-link">Vue</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/javascript/" class="nav-link">《JavaScript教程》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/es6/" class="nav-link">《ES6 教程》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/vue/" class="nav-link">《Vue》笔记</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/vdoing-demo-repository/note/wx-miniprogram/" class="nav-link">小程序笔记</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/vdoing-demo-repository/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/vdoing-demo-repository/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/vdoing-demo-repository/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><a href="/vdoing-demo-repository/archives/" class="nav-link">归档</a></div> <a href="https://github.com/xugaoyi/vuepress-theme-vdoing" target="_blank" rel="noopener noreferrer" class="repo-link">
    GitHub
    <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav>  <ul class="sidebar-links"><li><a href="/vdoing-demo-repository/pages/f344d070a1031ef7/" class="sidebar-link">ECMAScript 6 简介</a></li><li><a href="/vdoing-demo-repository/pages/c1edd70a6b7c7872/" class="sidebar-link">let 和 const 命令</a></li><li><a href="/vdoing-demo-repository/pages/b1ab10a62f7564da/" class="sidebar-link">变量的解构赋值</a></li><li><a href="/vdoing-demo-repository/pages/ca89eca8adeba5f4/" class="sidebar-link">字符串的扩展</a></li><li><a href="/vdoing-demo-repository/pages/a650b4a0ebfc9350/" class="sidebar-link">字符串的新增方法</a></li><li><a href="/vdoing-demo-repository/pages/0473261a6ab0ee8c/" class="sidebar-link">正则的扩展</a></li><li><a href="/vdoing-demo-repository/pages/5dfea9a0f2d1a392/" class="sidebar-link">数值的扩展</a></li><li><a href="/vdoing-demo-repository/pages/8ed309d668b20264/" class="sidebar-link">函数的扩展</a></li><li><a href="/vdoing-demo-repository/pages/e34009d60d8bc4b2/" class="sidebar-link">数组的扩展</a></li><li><a href="/vdoing-demo-repository/pages/b5e3e0a0ff6e9c25/" class="sidebar-link">对象的扩展</a></li><li><a href="/vdoing-demo-repository/pages/e85e68947502cf90/" class="sidebar-link">对象的新增方法</a></li><li><a href="/vdoing-demo-repository/pages/02c86eb2792f3262/" class="sidebar-link">Symbol</a></li><li><a href="/vdoing-demo-repository/pages/0c21dae358fca16b/" class="sidebar-link">Set 和 Map 数据结构</a></li><li><a href="/vdoing-demo-repository/pages/f56ec2ab97d60483/" class="sidebar-link">Proxy</a></li><li><a href="/vdoing-demo-repository/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/vdoing-demo-repository/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/vdoing-demo-repository/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/vdoing-demo-repository/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/vdoing-demo-repository/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/vdoing-demo-repository/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/vdoing-demo-repository/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/vdoing-demo-repository/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/vdoing-demo-repository/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/vdoing-demo-repository/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/vdoing-demo-repository/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/vdoing-demo-repository/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/vdoing-demo-repository/pages/16121351be68691b/" aria-current="page" class="active sidebar-link">异步遍历器</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/vdoing-demo-repository/pages/16121351be68691b/#同步遍历器的问题" class="sidebar-link">同步遍历器的问题</a></li><li class="sidebar-sub-header level2"><a href="/vdoing-demo-repository/pages/16121351be68691b/#异步遍历的接口" class="sidebar-link">异步遍历的接口</a></li><li class="sidebar-sub-header level2"><a href="/vdoing-demo-repository/pages/16121351be68691b/#for-await-of" class="sidebar-link">for await...of</a></li><li class="sidebar-sub-header level2"><a href="/vdoing-demo-repository/pages/16121351be68691b/#异步-generator-函数" class="sidebar-link">异步 Generator 函数</a></li><li class="sidebar-sub-header level2"><a href="/vdoing-demo-repository/pages/16121351be68691b/#yield-语句" class="sidebar-link">yield* 语句</a></li></ul></li><li><a href="/vdoing-demo-repository/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/vdoing-demo-repository/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/vdoing-demo-repository/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/vdoing-demo-repository/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/vdoing-demo-repository/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/vdoing-demo-repository/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/vdoing-demo-repository/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06225672><div class="articleInfo" data-v-06225672><ul class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/vdoing-demo-repository/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/vdoing-demo-repository/note/es6/#《ES6 教程》笔记" data-v-06225672>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="javascript:;" data-v-06225672>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">异步遍历器<!----></h1> <!----> <div class="theme-vdoing-content content__default"><h1 id="异步遍历器"><a href="#异步遍历器" class="header-anchor">#</a> 异步遍历器</h1> <h2 id="同步遍历器的问题"><a href="#同步遍历器的问题" class="header-anchor">#</a> 同步遍历器的问题</h2> <p>《遍历器》一章说过，Iterator 接口是一种数据遍历的协议，只要调用遍历器对象的<code>next</code>方法，就会得到一个对象，表示当前遍历指针所在的那个位置的信息。<code>next</code>方法返回的对象的结构是<code>{value, done}</code>，其中<code>value</code>表示当前的数据的值，<code>done</code>是一个布尔值，表示遍历是否结束。
</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> index<span class="token operator">++</span><span class="token punctuation">,</span> <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> it <span class="token operator">=</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 0</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 1</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value <span class="token comment">// 2</span>
<span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，变量<code>it</code>是一个遍历器（iterator）。每次调用<code>it.next()</code>方法，就返回一个对象，表示当前遍历位置的信息。</p> <p>这里隐含着一个规定，<code>it.next()</code>方法必须是同步的，只要调用就必须立刻返回值。也就是说，一旦执行<code>it.next()</code>方法，就必须同步地得到<code>value</code>和<code>done</code>这两个属性。如果遍历指针正好指向同步操作，当然没有问题，但对于异步操作，就不太合适了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
          <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> index<span class="token operator">++</span><span class="token punctuation">,</span> <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，<code>next()</code>方法返回的是一个 Promise 对象，这样就不行，不符合 Iterator 协议，只要代码里面包含异步操作都不行。也就是说，Iterator 协议里面<code>next()</code>方法只能包含同步操作。</p> <p>目前的解决方法是，将异步操作包装成 Thunk 函数或者 Promise 对象，即<code>next()</code>方法返回值的<code>value</code>属性是一个 Thunk 函数或者 Promise 对象，等待以后返回真正的值，而<code>done</code>属性则还是同步产生的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">{</span>
    <span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">{</span>
        <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token parameter">resolve</span> <span class="token operator">=&gt;</span> <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">resolve</span><span class="token punctuation">(</span>index<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
        <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">false</span>
      <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> it <span class="token operator">=</span> <span class="token function">idMaker</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>value<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">o</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
<span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，<code>value</code>属性的返回值是一个 Promise 对象，用来放置异步操作。但是这样写很麻烦，不太符合直觉，语义也比较绕。</p> <p>ES2018 <a href="https://github.com/tc39/proposal-async-iteration" target="_blank" rel="noopener noreferrer">引入<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>了“异步遍历器”（Async Iterator），为异步操作提供原生的遍历器接口，即<code>value</code>和<code>done</code>这两个属性都是异步产生。</p> <h2 id="异步遍历的接口"><a href="#异步遍历的接口" class="header-anchor">#</a> 异步遍历的接口</h2> <p>异步遍历器的最大的语法特点，就是调用遍历器的<code>next</code>方法，返回的是一个 Promise 对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>asyncIterator
  <span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>
    <span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span> value<span class="token punctuation">,</span> done <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token comment">/* ... */</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>asyncIterator</code>是一个异步遍历器，调用<code>next</code>方法以后，返回一个 Promise 对象。因此，可以使用<code>then</code>方法指定，这个 Promise 对象的状态变为<code>resolve</code>以后的回调函数。回调函数的参数，则是一个具有<code>value</code>和<code>done</code>两个属性的对象，这个跟同步遍历器是一样的。</p> <p>我们知道，一个对象的同步遍历器的接口，部署在<code>Symbol.iterator</code>属性上面。同样地，对象的异步遍历器接口，部署在<code>Symbol.asyncIterator</code>属性上面。不管是什么样的对象，只要它的<code>Symbol.asyncIterator</code>属性有值，就表示应该对它进行异步遍历。</p> <p>下面是一个异步遍历器的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

asyncIterator
<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult1</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult1<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: 'a', done: false }</span>
  <span class="token keyword">return</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult2</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: 'b', done: false }</span>
  <span class="token keyword">return</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">iterResult3</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>iterResult3<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { value: undefined, done: true }</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>上面代码中，异步遍历器其实返回了两次值。第一次调用的时候，返回一个 Promise 对象；等到 Promise 对象<code>resolve</code>了，再返回一个表示当前数据成员信息的对象。这就是说，异步遍历器与同步遍历器最终行为是一致的，只是会先返回 Promise 对象，作为中介。</p> <p>由于异步遍历器的<code>next</code>方法，返回的是一个 Promise 对象。因此，可以把它放在<code>await</code>命令后面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: 'a', done: false }</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: 'b', done: false }</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">await</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// { value: undefined, done: true }</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>next</code>方法用<code>await</code>处理以后，就不必使用<code>then</code>方法了。整个流程已经很接近同步处理了。</p> <p>注意，异步遍历器的<code>next</code>方法是可以连续调用的，不必等到上一步产生的 Promise 对象<code>resolve</code>以后再调用。这种情况下，<code>next</code>方法会累积起来，自动按照每一步的顺序运行下去。下面是一个例子，把所有的<code>next</code>方法放在<code>Promise.all</code>方法里面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> asyncIterable <span class="token operator">=</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> asyncIterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> v1<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token literal-property property">value</span><span class="token operator">:</span> v2<span class="token punctuation">}</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
  asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> asyncIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>v1<span class="token punctuation">,</span> v2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// a b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>另一种用法是一次性调用所有的<code>next</code>方法，然后<code>await</code>最后一步操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">runner</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> writer <span class="token operator">=</span> <span class="token function">openFile</span><span class="token punctuation">(</span><span class="token string">'someFile.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">await</span> writer<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">runner</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h2 id="for-await-of"><a href="#for-await-of" class="header-anchor">#</a> for await...of</h2> <p>前面介绍过，<code>for...of</code>循环用于遍历同步的 Iterator 接口。新引入的<code>for await...of</code>循环，则是用于遍历异步的 Iterator 接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>createAsyncIterable()</code>返回一个拥有异步遍历器接口的对象，<code>for...of</code>循环自动调用这个对象的异步遍历器的<code>next</code>方法，会得到一个 Promise 对象。<code>await</code>用来处理这个 Promise 对象，一旦<code>resolve</code>，就把得到的值（<code>x</code>）传入<code>for...of</code>的循环体。</p> <p><code>for await...of</code>循环的一个用途，是部署了 asyncIterable 操作的异步接口，可以直接放入这个循环。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> body <span class="token operator">=</span> <span class="token string">''</span><span class="token punctuation">;</span>

<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span><span class="token punctuation">(</span><span class="token keyword">const</span> data <span class="token keyword">of</span> req<span class="token punctuation">)</span> body <span class="token operator">+=</span> data<span class="token punctuation">;</span>
  <span class="token keyword">const</span> parsed <span class="token operator">=</span> <span class="token constant">JSON</span><span class="token punctuation">.</span><span class="token function">parse</span><span class="token punctuation">(</span>body<span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'got'</span><span class="token punctuation">,</span> parsed<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，<code>req</code>是一个 asyncIterable 对象，用来异步读取数据。可以看到，使用<code>for await...of</code>循环以后，代码会非常简洁。</p> <p>如果<code>next</code>方法返回的 Promise 对象被<code>reject</code>，<code>for await...of</code>就会报错，要用<code>try...catch</code>捕捉。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">createRejectingIterable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">error</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>注意，<code>for await...of</code>循环也可以用于同步遍历器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token punctuation">[</span><span class="token string">'a'</span><span class="token punctuation">,</span> <span class="token string">'b'</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>Node v10 支持异步遍历器，Stream 就部署了这个接口。下面是读取文件的传统写法与异步遍历器写法的差异。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 传统写法</span>
<span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token parameter">inputFilePath</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> readStream <span class="token operator">=</span> fs<span class="token punctuation">.</span><span class="token function">createReadStream</span><span class="token punctuation">(</span>
    inputFilePath<span class="token punctuation">,</span>
    <span class="token punctuation">{</span> <span class="token literal-property property">encoding</span><span class="token operator">:</span> <span class="token string">'utf8'</span><span class="token punctuation">,</span> <span class="token literal-property property">highWaterMark</span><span class="token operator">:</span> <span class="token number">1024</span> <span class="token punctuation">}</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
  readStream<span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'data'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">chunk</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'&gt;&gt;&gt; '</span><span class="token operator">+</span>chunk<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  readStream<span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'end'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'### DONE ###'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 异步遍历器写法</span>
<span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token parameter">inputFilePath</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> readStream <span class="token operator">=</span> fs<span class="token punctuation">.</span><span class="token function">createReadStream</span><span class="token punctuation">(</span>
    inputFilePath<span class="token punctuation">,</span>
    <span class="token punctuation">{</span> <span class="token literal-property property">encoding</span><span class="token operator">:</span> <span class="token string">'utf8'</span><span class="token punctuation">,</span> <span class="token literal-property property">highWaterMark</span><span class="token operator">:</span> <span class="token number">1024</span> <span class="token punctuation">}</span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> chunk <span class="token keyword">of</span> readStream<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'&gt;&gt;&gt; '</span><span class="token operator">+</span>chunk<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'### DONE ###'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div><h2 id="异步-generator-函数"><a href="#异步-generator-函数" class="header-anchor">#</a> 异步 Generator 函数</h2> <p>就像 Generator 函数返回一个同步遍历器对象一样，异步 Generator 函数的作用，是返回一个异步遍历器对象。</p> <p>在语法上，异步 Generator 函数就是<code>async</code>函数与 Generator 函数的结合。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">const</span> genObj <span class="token operator">=</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
genObj<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// { value: 'hello', done: false }</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>gen</code>是一个异步 Generator 函数，执行后返回一个异步 Iterator 对象。对该对象调用<code>next</code>方法，返回一个 Promise 对象。</p> <p>异步遍历器的设计目的之一，就是 Generator 函数处理同步操作和异步操作时，能够使用同一套接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 同步 Generator 函数</span>
<span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">iterable<span class="token punctuation">,</span> func</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> iter <span class="token operator">=</span> iterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>iterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token function">func</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token comment">// 异步 Generator 函数</span>
<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">iterable<span class="token punctuation">,</span> func</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> iter <span class="token operator">=</span> iterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token function">func</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，<code>map</code>是一个 Generator 函数，第一个参数是可遍历对象<code>iterable</code>，第二个参数是一个回调函数<code>func</code>。<code>map</code>的作用是将<code>iterable</code>每一步返回的值，使用<code>func</code>进行处理。上面有两个版本的<code>map</code>，前一个处理同步遍历器，后一个处理异步遍历器，可以看到两个版本的写法基本上是一致的。</p> <p>下面是另一个异步 Generator 函数的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">readLines</span><span class="token punctuation">(</span><span class="token parameter">path</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> file <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fileOpen</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">try</span> <span class="token punctuation">{</span>
    <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>file<span class="token punctuation">.</span><span class="token constant">EOF</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">yield</span> <span class="token keyword">await</span> file<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
    <span class="token keyword">await</span> file<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，异步操作前面使用<code>await</code>关键字标明，即<code>await</code>后面的操作，应该返回 Promise 对象。凡是使用<code>yield</code>关键字的地方，就是<code>next</code>方法停下来的地方，它后面的表达式的值（即<code>await file.readLine()</code>的值），会作为<code>next()</code>返回对象的<code>value</code>属性，这一点是与同步 Generator 函数一致的。</p> <p>异步 Generator 函数内部，能够同时使用<code>await</code>和<code>yield</code>命令。可以这样理解，<code>await</code>命令用于将外部操作产生的值输入函数内部，<code>yield</code>命令用于将函数内部的值输出。</p> <p>上面代码定义的异步 Generator 函数的用法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> line <span class="token keyword">of</span> <span class="token function">readLines</span><span class="token punctuation">(</span>filePath<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>line<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>异步 Generator 函数可以与<code>for await...of</code>循环结合起来使用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">prefixLines</span><span class="token punctuation">(</span><span class="token parameter">asyncIterable</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> line <span class="token keyword">of</span> asyncIterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token string">'&gt; '</span> <span class="token operator">+</span> line<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>异步 Generator 函数的返回值是一个异步 Iterator，即每次调用它的<code>next</code>方法，会返回一个 Promise 对象，也就是说，跟在<code>yield</code>命令后面的，应该是一个 Promise 对象。如果像上面那个例子那样，<code>yield</code>命令后面是一个字符串，会被自动包装成一个 Promise 对象。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> url <span class="token operator">=</span> <span class="token string">'https://www.random.org/decimal-fractions/'</span>
    <span class="token operator">+</span> <span class="token string">'?num=1&amp;dec=10&amp;col=1&amp;format=plain&amp;rnd=new'</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token function">fetch</span><span class="token punctuation">(</span>url<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Start'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// (A)</span>
  <span class="token keyword">yield</span> <span class="token string">'Result: '</span> <span class="token operator">+</span> <span class="token keyword">await</span> result<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// (B)</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> ag <span class="token operator">=</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
ag<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>上面代码中，<code>ag</code>是<code>asyncGenerator</code>函数返回的异步遍历器对象。调用<code>ag.next()</code>以后，上面代码的执行顺序如下。</p> <ol><li><code>ag.next()</code>立刻返回一个 Promise 对象。</li> <li><code>asyncGenerator</code>函数开始执行，打印出<code>Start</code>。</li> <li><code>await</code>命令返回一个 Promise 对象，<code>asyncGenerator</code>函数停在这里。</li> <li>A 处变成 fulfilled 状态，产生的值放入<code>result</code>变量，<code>asyncGenerator</code>函数继续往下执行。</li> <li>函数在 B 处的<code>yield</code>暂停执行，一旦<code>yield</code>命令取到值，<code>ag.next()</code>返回的那个 Promise 对象变成 fulfilled 状态。</li> <li><code>ag.next()</code>后面的<code>then</code>方法指定的回调函数开始执行。该回调函数的参数是一个对象<code>{value, done}</code>，其中<code>value</code>的值是<code>yield</code>命令后面的那个表达式的值，<code>done</code>的值是<code>false</code>。</li></ol> <p>A 和 B 两行的作用类似于下面的代码。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token function">fetchRandom</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">result</span> <span class="token operator">=&gt;</span> result<span class="token punctuation">.</span><span class="token function">text</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
  <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">result</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
     <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
       <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token string">'Result: '</span> <span class="token operator">+</span> result<span class="token punctuation">,</span>
       <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>如果异步 Generator 函数抛出错误，会导致 Promise 对象的状态变为<code>reject</code>，然后抛出的错误被<code>catch</code>方法捕获。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'Problem!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">asyncGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">err</span> <span class="token operator">=&gt;</span> console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// Error: Problem!</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>注意，普通的 async 函数返回的是一个 Promise 对象，而异步 Generator 函数返回的是一个异步 Iterator 对象。可以这样理解，async 函数和异步 Generator 函数，是封装异步操作的两种方法，都用来达到同一种目的。区别在于，前者自带执行器，后者通过<code>for await...of</code>执行，或者自己编写执行器。下面就是一个异步 Generator 函数的执行器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">takeAsync</span><span class="token punctuation">(</span><span class="token parameter">asyncIterable<span class="token punctuation">,</span> count <span class="token operator">=</span> <span class="token number">Infinity</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> iterator <span class="token operator">=</span> asyncIterable<span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span>asyncIterator<span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">while</span> <span class="token punctuation">(</span>result<span class="token punctuation">.</span>length <span class="token operator">&lt;</span> count<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> <span class="token punctuation">{</span>value<span class="token punctuation">,</span> done<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
    result<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> result<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，异步 Generator 函数产生的异步遍历器，会通过<code>while</code>循环自动执行，每当<code>await iterator.next()</code>完成，就会进入下一轮循环。一旦<code>done</code>属性变为<code>true</code>，就会跳出循环，异步遍历器执行结束。</p> <p>下面是这个自动执行器的一个使用实例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token string">'c'</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token keyword">return</span> <span class="token keyword">await</span> <span class="token function">takeAsync</span><span class="token punctuation">(</span><span class="token function">gen</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ['a', 'b', 'c']</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>异步 Generator 函数出现以后，JavaScript 就有了四种函数形式：普通函数、async 函数、Generator 函数和异步 Generator 函数。请注意区分每种函数的不同之处。基本上，如果是一系列按照顺序执行的异步操作（比如读取文件，然后写入新内容，再存入硬盘），可以使用 async 函数；如果是一系列产生相同数据结构的异步操作（比如一行一行读取文件），可以使用异步 Generator 函数。</p> <p>异步 Generator 函数也可以通过<code>next</code>方法的参数，接收外部传入的数据。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> writer <span class="token operator">=</span> <span class="token function">openFile</span><span class="token punctuation">(</span><span class="token string">'someFile.txt'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 立即执行</span>
writer<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token string">'world'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 立即执行</span>
<span class="token keyword">await</span> writer<span class="token punctuation">.</span><span class="token function">return</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 等待写入结束</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>openFile</code>是一个异步 Generator 函数。<code>next</code>方法的参数，向该函数内部的操作传入数据。每次<code>next</code>方法都是同步执行的，最后的<code>await</code>命令用于等待整个写入操作结束。</p> <p>最后，同步的数据结构，也可以使用异步 Generator 函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">createAsyncIterable</span><span class="token punctuation">(</span><span class="token parameter">syncIterable</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> elem <span class="token keyword">of</span> syncIterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">yield</span> elem<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，由于没有异步操作，所以也就没有使用<code>await</code>关键字。</p> <h2 id="yield-语句"><a href="#yield-语句" class="header-anchor">#</a> yield* 语句</h2> <p><code>yield*</code>语句也可以跟一个异步遍历器。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">yield</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
  <span class="token keyword">yield</span> <span class="token string">'b'</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">async</span> <span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">gen2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// result 最终会等于 2</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> <span class="token keyword">yield</span><span class="token operator">*</span> <span class="token function">gen1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，<code>gen2</code>函数里面的<code>result</code>变量，最后的值是<code>2</code>。</p> <p>与同步 Generator 函数一样，<code>for await...of</code>循环会展开<code>yield*</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">for</span> <span class="token keyword">await</span> <span class="token punctuation">(</span><span class="token keyword">const</span> x <span class="token keyword">of</span> <span class="token function">gen2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// a</span>
<span class="token comment">// b</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></div></div> <!----> <div class="page-edit"><div class="edit-link"><a href="https://github.com/xugaoyi/vuepress-theme-vdoing/edit/master/docs/《ES6 教程》笔记/27.异步遍历器.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----> <!----></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/vdoing-demo-repository/pages/32c35f7651d6e58e/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">读懂 ECMAScript 规格</div></a> <a href="/vdoing-demo-repository/pages/a2ba314746bfdbdd/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">ArrayBuffer</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
        ←
        <a href="/vdoing-demo-repository/pages/32c35f7651d6e58e/" class="prev">读懂 ECMAScript 规格</a></span> <span class="next"><a href="/vdoing-demo-repository/pages/a2ba314746bfdbdd/">ArrayBuffer</a>→
      </span></p></div></div></div> <!----></main></div> <div class="footer"><!----> 
  Theme by
  <a href="https://github.com/xugaoyi/vuepress-theme-vdoing" target="_blank" title="本站主题">Vdoing</a> 
    | Copyright © 2019-2023
    <span>Evan Xu | MIT License</span></div> <div class="buttons"><div title="返回顶部" class="button blur go-to-top iconfont icon-fanhuidingbu" style="display:none;"></div> <div title="去评论" class="button blur go-to-comment iconfont icon-pinglun" style="display:none;"></div> <div title="主题模式" class="button blur theme-mode-but iconfont icon-zhuti"><ul class="select-box" style="display:none;"><li class="iconfont icon-zidong">
          跟随系统
        </li><li class="iconfont icon-rijianmoshi">
          浅色模式
        </li><li class="iconfont icon-yejianmoshi">
          深色模式
        </li><li class="iconfont icon-yuedu">
          阅读模式
        </li></ul></div></div> <!----> <!----> <!----></div><div class="global-ui"></div></div>
    <script src="/vdoing-demo-repository/assets/js/app.52e6b5d0.js" defer></script><script src="/vdoing-demo-repository/assets/js/2.f10ddeba.js" defer></script><script src="/vdoing-demo-repository/assets/js/111.84bc1774.js" defer></script>
  </body>
</html>
